1. Field of the Invention
The present invention relates to a quadrature-quadrature amplitude modulation (Q.sup.2 AM) . Specifically, this invention is a Q.sup.2 AM method and apparatus for making a modulated signal have a constant envelope.
2. Discussion of Related Art
As information is getting important in the recent society, many communication terminals, such as a personal communication terminal and a mobile communication terminal, have been developed and spread. Since these communication terminals usually operate digitally, and transmit data through radio, they employ a digital modulation system which mixes digital data with a specified frequency signal, such as sine or cosine wave in signal transmission.
The conventional digital modulation system employed by a mobile or personal communication system is a quadrature phase shift keying (QPSK) system or a minimum shift keying (MSK) system. Under the present communication environment where frequency resource is limited, the modulation method described above cannot satisfy the requirements of high speed transmission and mass information transmission, such as image data transmission.
To solve these problems, a Q.sup.2 AM system, where bandwidth efficiency is improved, has been developed by combining two existing systems, a quadrature-quadrature PSK (Q.sup.2 PSK) system and a quadrature amplitude modulation (QAM) system.
FIG. 1 is a block diagram of a Q.sup.2 AM system according to prior art.
Serial-to-parallel converter 11 receives 8-bit data (m1, m2, m3, m4, m5, m6, m7, m8) in serial and outputs the data in parallel in the unit of 2-bit. Mappers 12 to 15 convert the respective 2-bit data, transmitted from serial-to-parallel converter 11, into level signals corresponding to relevant data values.
If it is assumed that odd input data is Mi, and even input data is Mj, mappers 12 to 15 generate specified level signals corresponding to input data values as shown in the following Table 1;
TABLE 1 ______________________________________ Mi Mj Output (a.sub.i) ______________________________________ 0 0 +3.beta. 0 1 +1.beta. 1 0 -1.beta. 1 1 -3.beta. ______________________________________
where .beta., as a parameter, is related to average signal energy E.sub.S by .sqroot.E.sub.S /10.
Mixers 16 to 19 respectively mix the level signals (.alpha..sub.1 to .alpha..sub.4), generated by mappers 12 to 15, with specified frequency signals S.sub.1 (t) to S.sub.4 (t)!. The frequency signals S.sub.1 (t) to S.sub.4 (t)! are respectively expressed as the following formulas 1 through 4. EQU S.sub.1 (t)=sin (.pi.t/2T) cos 2.pi.f.sub.C t Formula 1! EQU S.sub.2 (t)=cos (.pi.t/2T) cos 2.pi.f.sub.C t Formula 2! EQU S.sub.3 (t)=sin (.pi.t/2T) sin 2.pi.f.sub.C t Formula 3! EQU S.sub.4 (t)=cos (.pi.t/2T) sin 2.pi.f.sub.C t Formula 4!
Adder 20 sums up the frequency signals generated by mixers, 16 to 19, and generates a signal, S.sub.Q.sup.2.sub.AM (t)!. This signal is defined by the following formula 5. EQU S.sub.Q.sup.2.sub.AM (t)=.alpha..sub.1 .multidot.sin (.pi.t/2T) cos 2.pi.f.sub.C t+.alpha..sub.2 .multidot.cos (.pi.t/2T) cos 2.pi.f.sub.C t+.alpha..sub.3 .multidot.sin (.pi.t/2T) sin 2.pi.f.sub.C t+.alpha..sub.4 .multidot.cos (.pi.t/2T) sin 2.pi.f.sub.C t Formula 5!
In the Q.sup.2 AM system, 8-bit data is simultaneously modulated. When expressing a signal interval corresponding to one bit data as a Tb, each data bit is output for 8-Tb. During that time, the 8-bit data is input to serial-to-parallel converter 11.
FIG. 2 is a graph illustrating a frequency spectrum of a modulated signal, which will be transmitted through radio. As already known, when the period of the signal is T, the bandwidth of transmitting and receiving frequency is set to 1/T. The bandwidth of a signal according to the above Q.sup.2 AM system becomes 1/(8Tb). Therefore, the bandwidth efficiency in the Q.sup.2 AM system is quadruple 1/(2Tb) in the QPSK system, or octuple 1/Tb in the PSK system. Moreover, the Q.sup.2 AM system can improve the bandwidth efficiency of the QAM system by two times.
FIG. 3 is a block diagram of a receiver for receiving and demodulating the quadrature-quadrature amplitude modulated signal by the Q.sup.2 AM system.
Mixers 31 to 34 mix the received Q.sup.2 AM signal with the specified frequency signals, S.sub.1 (t) to S.sub.4 (t)!, used in the transmitter. Integrators 35 to 38 integrate the signals generated by mixers 31 to 34 during one symbol interval, or 8-Tb. Demappers 39 to 42 determine the levels of the integrated signals generated by Integrators 35 to 38 and generate 2-bit data corresponding to the relevant levels, respectively. Parallel-to-serial converter 43 receives the data bits from demappers 39 to 42 in parallel and outputs them in serial.
If the Q.sup.2 AM signal expressed in the formula 5 is received in such the configuration, the received signal is respectively mixed with the frequency signals, S.sub.1 (t) to S.sub.4 (t)! by mixers 31 to 34, which are identical with the signals mixed in the transmitter, and the mixed signals are integrated by integrators 35 to 38 during one symbol interval, 8-Tb, thus detecting the signal levels corresponding to the level signals generated by mappers 12 to 15 shown in FIG. 1. The level signals generated by integrators 35 to 38 are determined by demappers 39 to 42, thus detecting 2-bit data which corresponds to a relevant level signal according to a regulation as shown in the table 1. The data is then converted into serial data by parallel-to-serial converter 43, thus outputting the same 8-bit data that was forwarded by the transmitter.
However, the conventional Q.sup.2 AM system described above has the following problems.
When the data which is input to the modulator is "00110100 11101001 01010011 . . . " in the Q.sup.2 AM system, data "00 11 01 . . . " is applied to mapper 12 shown in FIG. 1, thus mixer 16 generates a signal .alpha..sub.1 S.sub.1 (t) shown in FIG. 4. Through the same method, mixers 17 to 19 respectively generate signals .alpha..sub.2 S.sub.2 (t), .alpha..sub.3 S.sub.3 (t) and .alpha..sub.4 S.sub.4 (t) as shown in FIG. 4. If the signals shown in FIG. 4 are summed up by adder 20, a signal S.sub.Q.sup.2 AM (t) shown in FIG. 4 is produced. The signal, S.sub.Q.sup.2.sub.AM (t)!, generated by adder 20, has different envelopes according to data to be modulated.
For radio communication, since it is necessary to amplify the level of a signal forwarded through an antenna, a high power amplifier must be installed at the front stage of the antenna. Especially, since data must be transmitted between a land station and an artificial satellite, a high power amplifier must be installed at the output stage in a satellite communication system.
Usually a class C amplifier is employed as the high power amplifier to increase electric power efficiency. Since an input-to-output characteristic in the class C amplifier is non-linear, the phase is deviated in accordance with the change of an amplitude signal when the amplitude of an input signal changes. This deteriorates the performance of the system. Therefore, the input signal to the high power amplifier, such as a class C amplifier, must have a constant envelope.
However, since the amplitudes of output signals change according to output data in the conventional Q.sup.2 AM system, the Q.sup.2 AM system cannot be used in a non-linear communication system even though it has high bandwidth efficiency.